Under current social needs, internal combustion engines having improved output energy, such as those of high-rotation high-output type and those of high-compression type including supercharging devices, have been developed in recent years. Therefore, piston rings used for those internal combustion engines are required to be up-graded in accordance with the development of internal combustion engines.
Further, fuels containing no lead have been recently employed from the viewpoint of prevention of environmental pollution, but lead-containing fuels are still employed in various countries. In cylinders of the internal combustion engines using those lead-containing fuels, a corrosive atmosphere such as HCl and H.sub.2 SO.sub.4 is strikingly produced, so that chrome-plated piston rings which are conventionally and widely used seriously suffer from abrasion of a chrome-plated layer on the sliding surface. For coping with this problem, those piston rings are subjected to a large thickness-chrome plating treatment, but the resulting piston rings are not always satisfactory with respect to manufacturing cost and productivity.
Furthermore, a piston ring whose slidable surface is subjected to a nitriding treatment to resist abrasion of the slidable surface have been also developed. Accordingly, enhancement of the abrasion resistance and the corrosion resistance are more highly demanded than before, and increase of piston rings of this type is expected. In addition to the enhancements of the abrasion resistance and the corrosion resistance, breakage resistance should be also improved, because the piston rings might be broken when they are used in the internal combustion engines of high-rotation and high-output type.
FIG. 16 shows a part of a compression ring 1 which is a conventional piston ring, and on a surface of a steel body 2 is formed a nitrided layer 3 by means of a nitriding treatment. In accordance with the nitriding treatment, a porous layer of high rigidity and brittleness (generally referred to as "white layer") is formed on a surface of the nitrided layer. This rigid and brittle porous layer is removed by subjecting a slidable layer 4 to the post treatment, whereby the piston ring can be commercialized.
An example of the composition of the steel body is as follows in terms of wt. %:
C: 0.60-0.70, PA0 Si: not more than 0.35, PA0 Mn: 0.20-0.50, PA0 Cr: 13.00-14.00, PA0 Mo: 0.20-0.40, PA0 Fe: residual portion, and inevitable impurity. PA0 C: 0.80 (0.85)-0.95, PA0 Si: 0.35-0.50, PA0 Mn: 0.25-0.40, PA0 Cr: 17.00-18.00, PA0 Mo: 1.00-1.25, PA0 V: 0.08-0.15, PA0 Fe: residual portion, and inevitable impurity. PA0 C: 0.87-0.93, PA0 Si: 0.20-0.40, PA0 Mn: 0.20-0.40, PA0 Cr: 21.00-22.00, PA0 Mo: 0.20-0.40, PA0 Ni: 0.90-1.10, PA0 Fe: residual portion, and inevitable impurity.
Another example of the composition of the steel body is as follows in terms of wt. %:
A further example of the composition of the steel body is as follows in terms of wt. %:
When the compression ring 1 is fitted into a piston groove of the piston in one of the above-mentioned internal combustion engines and is operated therein, the ring expands and contacts in the radial direction and collides with the wall of the piston groove repeatedly, and thereby cracks are produced in the nitrided layer 3 during the operation, resulting in cracks 5, 7 on the slidable surface 4 and the side surface 6. Enlargement of these cracks causes unfavorable phenomena such as totally dropping or partial dropping of the nitrided layer, whereby a peeled portion 8 occurs on the slidable surface 4. In the extreme case, these phenomena might cause scuffing or abnormal abrasion of the compression ring 1 or might cause even breakage of the compression ring 1. Further, the compression ring repeatedly collides with the piston groove owing to its expanding and contracting motions in the radial direction and vertical motions in the axial direction. Therefore, if a porous white layer of high rigidity and brittleness remains on the corner portions C or in the vicinity thereof, cracks are easily brought about on those portions. The cracks are enlarged from those portions, and finally the compression ring is broken off in many cases.